The gas analysis of benzene gas mixtures by means of gas detector tubes has hitherto been carried out colorimetrically on the basis of p-quinoid compounds with paraformaldehyde/sulfuric acid or with iodine pentoxide. The determination with I2O5 yields full, sharp display, which can be read by the user with certainty. The sensitivity of 2 ppm of benzene and the selectivity against, e.g., gasoline hydrocarbons are, however, insufficient.
A process for detecting benzene in gases, which comprises the reacting of the gas to be tested with a mixture of paraformaldehyde and anhydrous sulfuric acid, which said mixture is preferably applied to silica gel, is known from DE 865 384 B. The sensitivity of the detection reaction can be increased by adding dimethyl sulfate. The detection on the basis of paraformaldehyde/sulfuric acid is selective and sensitive enough in its application with a detection limit of about 0.5 ppm of benzene. However, the display accuracy is insufficient because of a diffuse change in color, so that the user can read the concentration displayed in an only poorly defined manner. In addition, the measurement times with 20 minutes and 20 strokes are not user-friendly and not accepted in the container fumigation market. Said tubes cannot be used for release measurements, e.g., in the container fumigation market because of poor sensitivity, reading accuracy, measurement time and selectivity. A qualitative measurement, which provides information on whether a container can be safely accessed or not, is required for this application.